Various methods for video recording apparatuses and others for contactless measurement of a depth in a three-dimensional scene, that is, a distance to each object have been proposed. The methods can be broadly divided into two, namely, (i) methods for actively calculating the distance based on an arrival time or an angle of reflected waves after irradiating an object with, for example, infrared rays, ultrasound, and laser light and (ii) methods for passively calculating the distance based on an image of an object. Particularly, the passive methods that do not require any device that irradiates an object with infrared rays or others have widely been applied to cameras.
Out of many of the passive methods that have been proposed, Depth from Defocus (abbreviated as “DFD” hereinafter) is a method for measuring a distance to an object based on blur information which varies in size and shape according to the distance to the object. The DFD features, for example, measuring a distance to an object, using the less number of images captured by a single camera.
The following simply describes the principle of the DFD.
The DFD is a method for measuring a distance to an object using images having different focal points, based on blur information. A blurred image is considered as an image obtained by convolving a point spread function (abbreviated as “PSF” hereinafter) that is a function representing an object distance, into an all-in-focus image having no blur caused by the lens. Since the PSF is a function representing an object distance, the object distance can be computed in the DFD by estimating blur from the blurred captured image. Here, the all-in-focus image and the object distance are unknowns. An equation expressing a blurred image, an all-in-focus image, and an object distance holds for one blurred image. Thus, new blurred images having different focal points are captured, and the corresponding equations are obtained. The obtained equations are solved to find object distances. Various proposals on the DFD including Patent Literature (PTL) 1 have been made as methods for obtaining and solving such equations.